Many products are subjected to mechanical vibrations during their lifetimes, and vibration testing of products during the design and manufacturing stages has proven very valuable to improve their expected lifetimes.
The experienced real world vibration usually includes all six degrees of freedom (DOF), that is, the vibration usually has linear acceleration components along the three orthogonal axes, and angular acceleration components about those axes. The best simulation of the vibration environment would include vibration in all six degrees of freedom.
Several vibration test machines have been designed to produce six degrees of freedom vibration. They often use hydraulic actuators to move a test table, on which the test article is mounted. Since each actuator mechanism usually produces motion in a single degree of freedom, each actuator must be coupled to the table with bearings that have five degrees of freedom. In this way, the actuator controls the single degree of freedom of the point on the table where its drive link is connected. The five DOF bearings allow motion in the other five degrees of freedom. With six actuators properly arranged, each controlling a single degree of freedom, all six degrees of freedom of the table can be controlled.
Many actuator and bearing configurations are possible to achieve six degrees of freedom vibration and/or shock. Spherical bearings at each end of the actuator, links with spherical bearings placed between the actuator and the table, or special five degree of freedom bearings that comprise both a sliding and rotating bearing in a single element are examples.
These systems are limited in their frequency response by the dynamics of the hydraulic shaker, the table, and any connecting links. A very fundamental limitation to the frequency response comes from the hydraulic shaker.
Typical hydraulic servo valves are limited to 50 or 100 Hz frequency response. U.S. Pat. No. 5,343,752, assigned to Team Corporation, discloses a servo valve and double-acting piston actuator that responds to 1000 to 2000 Hz depending on the size of the actuator. Several multi-degree of freedom systems are disclosed in that patent. Each uses a high frequency valve to produce higher vibration frequencies than previously attainable by a multiple degree of freedom shaker. One of those systems, referred to as the Cube test system, comprises actuators on the inside of the vibration table. This system improves on the frequency response of a six DOF vibration test system, raising the controllable frequency from about 50 Hz for the prior art, to about 250 Hz. Even greater frequency response is desired, so the Cube type system forms the basis upon which the improvements of the present invention are compared.
Compared to the Cube style multi-axis vibration table, the present invention has higher frequency response and better table uniformity (less distortion). Electro-dynamic (ED) actuators have better frequency response and better freedom from distortion than even the best electro-hydraulic (EH) shakers; and the smaller, stiffer table of the present invention provides a higher first mode frequency than the Cube test system. This translates to much higher G levels for the tests object.
The actuators of the Cube shaker are located inside the vibration table and are mounted to the reaction mass on legs that protrude through the bottom of the vibration table. It has been found that the mounting structure (legs) for the internal actuators has a relatively low natural frequency that limits the frequency response of that design to frequencies below what the actuators are capable of.
In addition, because the Cube shaker is carried (and vibrates) well above the reaction mass, large moments are generated that must be reacted to by the reaction mass. Keeping reaction mass motion to a minimum requires use of large reaction masses.